Ignition coils are known for use in connection with an internal combustion engine such as an automobile engine, and which include a primary winding, a secondary winding, and a magnetic circuit. The magnetic circuit conventionally may comprise a cylindrical-shaped, central core extending along an axis, located radially inwardly of the primary and secondary windings and magnetically coupled thereto. The components are contained in a case formed of electrical insulating material, with an outer core or shield located outside of the case. One end of the secondary winding is conventionally configured to produce a relatively high voltage when a primary current through the primary winding is interrupted. The high voltage end is coupled to a spark plug, as known, that is arranged to generate a discharge spark responsive to the high voltage. It is further known to provide relatively slender ignition coil configuration that is adapted for mounting directly above the spark plug-commonly referred to as a “pencil” coil.
FIG. 1 illustrates a conventional secondary spool 28 on which a secondary coil 30 is wrapped or wound. Spool 28 includes opposing flanges 28a and 28b extending outwardly at approximately a 90 degree angle from each end of a main, cylindrical winding section 28c. Main winding section 28c carries the secondary coil 30. The secondary coil 30 is wound in a progressive fashion at a predetermined angle (after an initial “wedge” 30a is formed). The secondary coil is thus formed in a plurality of “layers” 30b that slant or are inclined relative to the main winding surface 28c. Each “layer” 30b has a certain number of turns. For reference, the high voltage end of the secondary coil is designated 30HV.
One problem in the design of ignition coils, particularly pencil coils, involves a relatively high voltage in the secondary coil near the high voltage end of the secondary spool. Applicants have determined that there are two main contributors to the high voltage: (1) a reflected voltage and (2) a magnetically induced voltage.
FIG. 2 shows the two components resolved, one from another, for an exemplary ignition coil. In an ignition coil, when the spark gap breaks down due to the application of the spark firing voltage thereacross, a relatively high voltage gradient is seen as the end of the coil connected to the spark plug. The magnitude of this voltage gradient is proportional to the current pulse flowing into the ignition coil from the breakdown of the gap (i.e., from ground, across the spark gap, and into the spark voltage end of the secondary coil). This component of the voltage will be referred to as a “reflected” voltage, and is designated as trace 26a in FIG. 2. It has been observed by Applicants that increases in the impedance between the ignition coil (i.e., particularly the secondary coil thereof and the spark plug gap tend to decrease the voltage gradient in the ignition coil. Therefore, as ignition coils are moved closer and closer to the spark plug (i.e., a coil-on-plug type versus a separate mount type ignition coil coupled through a spark plug cable, for instance), the level of the voltage gradient increases. The highest gradient is exhibited on the turns of the secondary winding closes to the spark gap. The gradient decreases as it propagates through the secondary winding. In addition, a component of the voltage in the secondary winding is magnetically-induced, with the highest gradient occurring in the middle of the longitudinal length of the secondary winding where the magnetic flux is the most concentrated. The magnetically-induced component is designated as trace 26b in FIG. 2.
FIG. 3 shows the superposition of these two influences, designated as trace 26c, when the spark plug is fired to produce a spark. Trace 26c shows the wire to wire voltage as a function of the distance (i.e., axial distance) from the high voltage (HV) end of the secondary coil. For reference, an open circuit trace 26d is also shown, which excludes the influence of the spark current pulse and the associated reflected voltage.
While the secondary winding 30 generally includes a thin film insulation of a type known in the art, such insulation does have its limits. The relatively high voltage between the windings can result in wire-to-wire shorts, causing the ignition coil to perform unsatisfactorily or even fail.
It is known to taper the radial thickness of the secondary winding (and thus the number of turns from the high-voltage (HV) end of the secondary winding towards the low voltage (LV) end of the secondary coil, in an effort to reduce the number of turns per layer, and accordingly the wire to wire voltage. However, this approach results in an unacceptably long taper distance not desirable for commercial products. In addition, it is known to provide a secondary coil spool having ramps on both ends, as seen by reference to U.S. Pat. No. 6,276,348 entitled “IGNITION COIL ASSEMBLY WITH SPOOL HAVING RAMPS AT BOTH ENDS THEREOF” issued to Skinner et al.
Accordingly, there is a need for an improved ignition apparatus that minimizes or eliminates one or more of the problems as set forth above.